Because Zidane is a hidden 5*base. More seriously, because they know Zidane will bring in more cash, and that few people will actually pull hard on this banner. Best use of Zidane is : his ff banner when his *form comes out, and people choice banners as he will always be in there with 2 5*base, 2 4*base and so on, and with a lot of money dropped into.
edit : random conspiracy. Go with it.

Should one spend the remaining 20 or so tickets they had chasing Onion Knight or Gilgamesh? I have neither, and my 2B/Orlandeau/Fryevia would kill for their first Genji Glove...but then, OK is calling to me with his chaining and DPS and elemental support...(PS - Doing well with a ready supply of Abels and Chizurus and their respective TMs already)...

Should one spend the remaining 20 or so tickets they had chasing Onion Knight or Gilgamesh? I have neither, and my 2B/Orlandeau/Fryevia would kill for their first Genji Glove...but then, OK is calling to me with his chaining and DPS and elemental support...(PS - Doing well with a ready supply of Abels and Chizurus and their respective TMs already)...

Click to expand...

2B doesn't need the Genji Glove, she has innate DW... but Landu and Fry would just love it, true enough. I never got Gilga myself, so I will do at least dailies here to get a GG for my Landu or Noctis.

This is one worst banners ever. Yeah Gilgemesh is amazing for Genji Gloves but your talking 1% chance to pull him while 19% chance to pull other 2 useless units on this banner no thanks will not pull on this banner at all.